Meanwhile, in a medium bowl, stir together ricotta cheese and 1 cup of mozzarella cheese.

Spoon 1/3 of beef mixture into a 5-quart slow cooker. Break 3 lasagna sheets in half and arrange over beef mixture; top with half of ricotta mixture. Repeat with another layer and finish with remaining 1/3 of beef mixture.

Cover slow cooker and cook on low setting for 4 to 6 hours. Remove cover; turn off heat and season to taste, if desired.

In a small bowl, combine remaining 1/2 cup of mozzarella cheese and Parmesan cheese; sprinkle over beef mixture. Cover and set aside until cheese melts and lasagna firms up, about 10 minutes.

I've always done mine in the oven with bacon bits, kabana or polish sausage on top of the cheese.
Though I find that if you make the sauce a few days in advance and store it in the fridge the flavours develop and you end up with a to-die-for lasagna.